The ferrate ion, FeO42−, is a tetrahedral ion that is believed to be isostructural with chromate, CrO42−, and permanganate, MnO4−. The ferrate ion has been suggested to exist in aqueous media as the tetrahedral species FeO42−. Redox potentials for FeO42− ion have been estimated in both acidic and basic media (R. H. Wood, J. Am. Chem. Soc., Vol. 80, p. 2038-2041 (1957)):FeO42−+8H++3e−→Fe3++4H2O E°=2.20VFeO42−+4H2O+3e−→Fe3++8OH− E°=0.72V
Ferrate is a strong oxidant that can react with a variety of inorganic or organic reducing agents and substrates (R. L. Bartzatt, J. Carr, Trans. Met. Chem., Vol. 11 (11), pp. 414-416 (1986); T. J. Audette, J. Quail, and P. Smith, J. Tetr. Lett., Vol. 2, pp. 279-282 (1971); D. Darling, V. Kumari, and J. BeMiller, J. Tetr. Lett., Vol. 40, p. 4143 (1972); and R. K. Murmann and H. J. Goff, J. Am. Chem. Soc., Vol. 93, p. 6058-6065 (1971)). It can, therefore, act as a selective oxidant for synthetic organic studies and is capable of oxidizing/removing a variety of organic and inorganic compounds from, and of destroying many contaminants in, aqueous and non-aqueous media.
In the absence of a more suitable reductant, ferrate will react with water to form ferric ion and molecular oxygen according to the following equation (J. Gump, W. Wagner, and E. Hart, Anal. Chem., Vol. 24., p. 1497-1498 (1952)).4FeO42−+10H2O→4 Fe3++20 OH−+3O2 
This reaction is of particular interest to water treatment because it provides a suitable mechanism for self-removal of ferrate from solution. In all oxidation reactions, the final iron product is the non-toxic ferric ion which forms hydroxide oligomers. Eventually flocculation and settling occur which remove suspended particulate matter.
The use of ferrate may therefore provide a safe, convenient, versatile and cost effective alternative to current approaches for water, wastewater, and sludge treatment. In this regard, ferrate is an environmentally friendly oxidant that represents a viable substitute for other oxidants, particularly chromate and chlorine, which are of environmental concern. Ferric oxide, typically known as rust, is the iron product of oxidation by ferrate. Therefore, ferrate has the distinction of being an “environmentally safe” oxidant. Although the oxidation reactions with ferrate appear similar to those known for MnO4− and CrO42−, ferrate exhibits greater functional group selectivity with higher rate of reactivity in its oxidations and generally reacts to produce a cleaner reaction product.
One problem hindering ferrate implementation is difficulty in its preparation. This difficulty may lead to increased production costs. Moreover, in addition to cost, the current methods known for producing a commercially useful and effective ferrate product, and the results of these methods, have been less than satisfactory. There exists a need for new synthetic preparative procedures that are easier and less expensive in order to provide ferrate material at economically competitive prices.
Three approaches for ferrate synthesis are known: electrolysis, oxidation of Fe2O3 in an alkaline melt, or oxidation of Fe(III) in a concentrated alkaline solution with a strong oxidant.
In the laboratory, by means of hypochlorite oxidation of iron (Fe(III)) in strongly alkaline (NaOH) solution, the ferrate product has been precipitated by the addition of saturated KOH (G. Thompson, L. Ockerman, and J. Schreyer, J. Am. Chem. Soc., Vol. 73, pp. 1379-81 (1951)):2Fe3++3OCl−+10OH−→2FeO42−+3Cl−+5H2OThe resulting purple solid is stable indefinitely when kept dry.
Commercial production of ferrate typically uses a synthetic scheme similar to the laboratory preparation, also involving a hypochlorite reaction. Most commonly, using alkaline oxidation of Fe(III), potassium ferrate (K2FeO4) is prepared via gaseous chlorine oxidation in caustic soda of ferric hydroxide, involving a hypochlorite intermediate. Another method for ferrate production was described by Johnson in U.S. Pat. No. 5,746,994.
A number of difficulties are associated with the production of ferrate using the method described above. For example, several requirements for reagent purity must be ensured for maximized ferrate yield and purity. However, even with these requirements satisfied, the purity of the potassium ferrate product still varies widely and depends upon many factors, such as reaction time, temperature, purity of reagents, and isolation process. Ferrate prepared this way generally contains impurities, with the major contaminants being alkali metal hydroxides and chlorides and ferric oxide. However, samples of this degree of purity are unstable and readily decompose completely into ferric oxides.
Other than the specific problems with product impurities and instability, there also exist mechanical problems associated with the isolation of the solid ferrate product, such as filtering cold lye solutions having a syrupy consistency.
Other processes for preparation of ferrates are known and used, many of them also involving the reactions with hypochlorite. For example, U.S. Pat. No. 5,202,108 to Deininger discloses a process for making stable, high-purity ferrate (VI) using beta-ferric oxide (beta-Fe2O3) and preferably monohydrated beta-ferric oxide (beta-Fe2O3.H2O), where the unused product stream can be recycled to the ferrate reactor for production of additional ferrate.
U.S. Pat. Nos. 4,385,045 and 4,551,326 to Thompson disclose a method for direct preparation of an alkali metal or alkaline earth metal ferrates from inexpensive, readily available starting materials, where the iron in the product has a valence of +4 or +6. The method involves reacting iron oxide with an alkali metal oxide or peroxide in an oxygen free atmosphere or by reacting elemental iron with an alkali metal peroxide in an oxygen free atmosphere.
U.S. Pat. No. 4,405,573 to Deininger et al. discloses a process for making potassium ferrate in large-scale quantities (designed to be a commercial process) by reacting potassium hydroxide, chlorine, and a ferric salt in the presence of a ferrate stabilizing compound.
U.S. Pat. No. 4,500,499 to Kaczur et al. discloses a method for obtaining highly purified alkali metal or alkaline earth metal ferrate salts from a crude ferrate reaction mixture, using both batch and continuous modes of operation.
U.S. Pat. No. 4,304,760 to Mein et al. discloses a method for selectively removing potassium hydroxide from crystallized potassium ferrate by washing it with an aqueous solution of a potassium salt (preferably a phosphate salt to promote the stability of the ferrate in the solid phase as well as in aqueous solution) and an inorganic acid at an alkaline pH.
U.S. Pat. No. 2,758,090 to Mills et al. discloses a method of making ferrate, involving a reaction with hypochlorite, as well as a method of stabilizing the ferrate product so that it can be used as an oxidizing agent.
U.S. Pat. No. 2,835,553 to Harrison et al. discloses a method, using a heating step, where novel alkali metal ferrates with a valence of +4 are prepared by reacting the ferrate (III) of an alkali metal with the oxide (or peroxide) of the same, or a different, alkali metal to yield the corresponding ferrate (IV).
U.S. Pat. No. 5,284,642 to Evrard et al. discloses the preparation of alkali or alkaline earth metal ferrates that are stable and industrially usable as oxidizers, and the use of these ferrates for water treatment by oxidation. Sulfate stabilization is also disclosed.
The development of an economical source of ferrate is desired to derive the benefits associated with ferrate application in a wide range of processes. In view of the difficulties associated with the previously known methods for preparing ferrates and the problems inherent in the ferrate produced by these known methods, there is therefore an existing need for a new preparative method for ferrate that is easy, convenient, safe and inexpensive, and that avoids both the chemical and mechanical problems. There also exists a need for a system which reduces or counteracts the limited stability of ferrate, and systems which employ ferrate as an environmentally friendly oxidant and disinfectant.